Axial-flow pump

ABSTRACT

An axial-flow pump employs an increased boss ratio Dh/Dt, wherein Dh is the hub diameter of a rotary shaft of a rotor, and Dt is the tip diameter of impeller vanes of the rotor. Specifically, the boss ratio is 0.65 to 0.85, preferably 0.7 to 0.8.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an axial-flow pump having a stablepressure characteristic; for example, an axial-flow pump that can befavorably applied as an artificial heart pump.

2. Description of the Related Art

Conventionally, pumps of various kinds, such as pulsating pumps, turbopumps, and roller pumps, have been used as artificial heart pumps. Turbopumps are suited for reduction in size. Particularly, axial-flow pumpsare most suited for reduction in size.

FIG. 4 is a schematic, sectional, structural view of a conventionalaxial-flow pump. As shown in FIG. 4, the axial-flow pump includes acylindrical housing 1; a rotor 3, which is an impeller and is rotatablysupported in the housing 1 in such a manner as to rotate about the axisX of the housing 1; and a drive mechanism for rotating the rotor 3.Rotation of the rotor 3 propels blood under pressure in the axialdirection (in FIG. 4, from right to left). In FIG. 4, inline arrows showa major blood flow path.

The specific configuration of the conventional axial-flow pump will bedescribed. A plurality of straightening vanes 4 are projectingly joinedto the inner wall surface of the housing 1 at a position locatedupstream of the rotor 3. A cylindrical, upstream stationary member 5 isjoined to the radially inner edges of the straightening vanes 4coaxially with the axis X. A plurality of plate-like blades of adiffuser 6 are projectingly joined to the inner wall surface of thehousing 1 at a position located downstream of the rotor 3. Acylindrical, downstream stationary member 7 is joined to the radiallyinner edges of the blades of the diffuser 6 coaxially with the axis X.The upstream end of the upstream stationary member 5 assumes the form ofa round nose so as to smoothly divide and lead blood to thestraightening vanes 4. The downstream end of the downstream stationarymember 7 assumes the form of a round nose so as to smoothly mergedivided flows of blood from the diffuser 6.

The downstream stationary member 7 contains a motor 10 (drivemechanism), which rotates about the axis X. A rotary shaft 8 is coupledwith the motor 10, so that the rotary shaft 8 rotates about the axis X.A plurality of impeller vanes 9 are projectingly joined to the outercircumferential surface of the rotary shaft 8. The radially outer edgesof the impeller vanes 9 closely oppose the inner wall surface of thehousing 1. The rotary shaft 8 and the impeller vanes 9 constitute therotor 3.

When the motor 10 is energized, the rotary shaft 8 and the impellervanes 9, which constitute the rotor 3, rotate unitarily about the axis Xin the interior of the housing 1. Accordingly, blood is taken into thehousing 1, its flow is straightened by the straightening vanes 4, andits pressure is increased by the impeller vanes 9, whereby the bloodbecomes blood having dynamic pressure. Then, the diffuser 6 causes mostof the blood having dynamic pressure to be restored to blood havingstatic pressure, which is discharged from the housing 1. In this manner,the axial pump propels blood under pressure.

However, detailed study on the discharge pressure characteristic of theaxial-flow pump have revealed that the discharge pressure characteristicis still not as desired. FIG. 5 is a schematic, partial, structural viewof the conventional axial-flow pump. The outside diameter of the rotaryshaft 8 of the rotor is called a hub diameter Dh; the outside diameterof the impeller vanes 9 of the rotor is called a tip diameter Dt; andtheir ratio Dh/Dt is called a boss ratio. In the conventional axial-flowpump, the boss ratio of the rotor is at most 0.5. Generally, the bossratio tends to be further reduced in order to improve power efficiency,air diffusion efficiency, water supply efficiency, or a like factor(refer to Japanese Patent Application Laid-Open (kokal) Nos. H08-33896and H05-253592).

FIG. 6 is a pair of conceptual views showing how fluid flows along therotor (impeller) of the conventional axial-flow pump. As shown in FIG.6(a), when the boss ratio is low; i.e., when the impeller vanes 9 occupya large portion of the rotor, reverse flows arise in an outercircumferential portion of the inlet region of the impeller vanes 9 inoperation at low flow rate. As a result, the pump head drops.

As shown in FIG. 6(b), when the flow rate lowers further from the levelof FIG. 6(a), reverse flows arise not only in an outer circumferentialportion of the inlet region of the impeller vanes 9, but also in aninner circumferential portion of the outlet region of the impeller vanes9. As a result, a main flow of fluid is biased toward the outercircumference, whose rotational speed is high, of the impeller vanes 9,so that the centrifugal effect, as seen in a centrifugal pump or thelike, causes an increase in pump head.

FIG. 7 is a graph showing a flow rate vs. pump head characteristic ofthe conventional axial-flow pump. As shown in FIG. 7, in operation at aflow rate lower than the design flow rate (a design point of operation),reverse flows in the inlet region cause a drop in pump head (see FIG.6(a)), or the centrifugal effect causes an increase in pump head (seeFIG. 6(b)). As a result, the axial-flow pump exhibits an unstable flowrate vs. pump head characteristic curve. Particularly, the centrifugaleffect tends to greatly increase the non-discharge pump head as comparedwith the rated pump head (a pump head at the design point of operation).

As mentioned above, because of an unstable pressure characteristic, theconventional axial-flow pump has encountered difficulty in applicationto such a working condition as to require constant discharge pressureregardless of flow rate (for example, use as an artificial heart pump).

SUMMARY OF THE INVENTION

In view of the foregoing, an object of the present invention is toprovide an axial-flow pump having a stable pressure characteristic.

To achieve the above object, the present invention provides anaxial-flow pump for propelling fluid under pressure by means of impellervanes, the axial-flow pump having such a boss ratio as to suppressbiased flow of the fluid caused by reverse flows arising in an outercircumferential portion of an inlet region of the impeller vanes andreverse flows arising in an inner circumferential portion of an outletregion of the impeller vanes.

Preferably, the boss ratio is 0.65 to 0.85.

The present invention further provides an axial-flow pump comprising ahousing, an impeller provided rotatably in the housing, and a drivemechanism for rotating the impeller, and adapted to axially propel fluidunder pressure by means of the drive mechanism rotating the impeller. Inthe axial-flow pump, the drive mechanism is fixedly attached to adownstream stationary member which, in turn, is fixedly attached to adiffuser located downstream of the impeller and projecting from an innerwall surface of the housing; the impeller comprises a rotary shaftcoupled with the drive mechanism and impeller vanes projecting from theouter circumferential surface of the rotary shaft; and a boss ratio,which is the ratio between the outside diameter of the rotary shaft andthe outside diameter of the impeller vanes, is 0.65 to 0.85.

The axial-flow pump of the present invention can exhibit a stablepressure characteristic and can reduce the degree of change in dischargepressure associated with a change in flow rate. Accordingly, theaxial-pump of the present invention can be applied as, for example, anartificial heart pump, which requires a stable discharge pressurecharacteristic. Furthermore, application of the axial-flow pump of thepresent invention as an artificial heart pump can implement anartificial heart pump of reduced size.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features and many of the attendant advantages ofthe present invention will be readily appreciated as the same becomesbetter understood by reference to the following detailed description ofthe preferred embodiment when considered in connection with theaccompanying drawings, in which:

FIG. 1 is a schematic, partial, structural view of an axial-flow pumpaccording to an embodiment of the present invention;

FIG. 2 is a pair of conceptual views showing how fluid flows along therotor (impeller) of the axial-flow pump according to the embodiment;

FIG. 3 is a graph showing a flow rate vs. pump head characteristic ofthe axial-flow pump according to the embodiment;

FIG. 4 is a schematic, sectional, structural view of a conventionalaxial-flow pump;

FIG. 5 is a schematic, partial, structural view of the conventionalaxial-flow pump;

FIG. 6 is a pair of conceptual views showing how fluid flows along therotor (impeller) of the conventional axial-flow pump; and

FIG. 7 is a graph showing a flow rate vs. pump head characteristic ofthe conventional axial-flow pump.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will next be described in detailwith reference to the drawings.

An axial-flow pump according to the present embodiment, whose schematic,partial structure is shown in FIG. 1, has the same structure as that ofthe conventional axial-flow pump shown in FIG. 4. Thus, repeateddescription of the structure is omitted. As compared with theconventional axial-flow pump, the axial-flow pump of the presentembodiment has a higher boss ratio. FIG. 1 corresponds to FIG. 5 andshows a partial, structural arrangement of a diffuser 6′, a downstreamstationary member 7′, a rotary shaft 8′, and impeller vanes 9′ in theaxial-flow pump of the present embodiment.

In the present embodiment, the rotary shaft 8′ and the impeller vanes 9′constitute a rotor, and the ratio between the hub diameter Dh of therotary shaft 8′ and the tip diameter Dt of the impeller vanes 9′; i.e.,the boss ratio Dh/Dt, is 0.65 to 0.85.

As shown in FIG. 2, even at a high boss ratio of 0.65 to 0.85; i.e.,even when a region of the rotor that is occupied by the impeller vanes9′ is relatively small, reverse flows arise in an outer circumferentialportion of the inlet region of the impeller vanes 9′ in operation at lowflow rate (FIG. 2(a)). As a result, as shown in FIG. 3, even in thepresent embodiment, the pump head drops.

When the flow rate lowers further from the level of FIG. 2(a), reverseflows arise not only in an outer circumferential portion of the inletregion of the impeller vanes 9′, but also in an inner circumferentialportion of the outlet region of the impeller vanes 9′ (FIG. 2(b)). As aresult, a main flow of fluid is biased toward the outer circumference,whose rotational speed is high, of the impeller vanes 9′, so that thecentrifugal effect causes an increase in pump head. However, since theboss ratio is relatively high, the biasing degree of fluid flow (thedegree of a change in fluid flow associated with a change in flow rate)is small as compared with the conventional axial-flow pump, as shown inFIG. 2(b). This indicates that the axial-flow pump of the presentembodiment suppresses occurrence of the centrifugal effect.

A boss ratio of 0.65 or higher, preferably 0.7 or higher, is effectivefor suppressing occurrence of the centrifugal effect and thus forstabilizing the pressure characteristic of an axial-flow pump. As shownin FIG. 3, the axial-flow pump having such a boss ratio exhibits astable pressure characteristic over a wide flow rate range including adesign flow rate (a design point of operation).

As the boss ratio increases, the centrifugal effect is suppressed morereliably, and thus a pressure characteristic becomes stabler. However,increasing the boss ratio reduces the cross-sectional area of a fluidflow path in the axial-flow pump. Thus, at high flow rate, friction lossincreases, causing a drop in pump head. As a result, when the boss ratiobecomes too high, the ratio between the non-discharge pump head and therated pump head increases, and thus a pressure characteristic becomesunstable. Therefore, the boss ratio must be 0.85 or less, preferably 0.8or less.

In selection of the boss ratio Dh/Dt, there are selected the hubdiameter Dh of the rotary shaft 8′ of the rotor and the tip diameter Dtof the impeller vanes 9′ of the rotor. In order to select a high bossratio, the hub diameter Dh and the tip diameter Dt are selected in thefollowing manners: while the tip diameter Dt is held constant, the hubdiameter Dh is increased; while the hub diameter Dh is held constant,the tip diameter Dt is decreased; both of the hub diameter Dh and thetip diameter Dt are increased in such a manner that the hub diameter Dhis increased at a higher rate than the tip diameter Dt; and both of thehub diameter Dh and the tip diameter Dt are decreased in such a mannerthat the tip diameter Dt is decreased at a higher rate than the hubdiameter Dh. A high boss ratio may be selected in any of the abovemanners in view of the degree of the above-mentioned effect of anincrease in the boss ratio, the size of an axial-flow pump, and thecross-sectional area of a fluid flow path in the axial-flow pump.

For example, in design of an axial-flow pump, when the hub diameter Dhis increased while the tip diameter Dt is held constant (first manner),there is obtained a small-sized axial-flow pump that is equivalent insize to the conventional axial-flow pump while having a high boss ratioand a stable pressure characteristic. In this case, since thecross-sectional area of a fluid flow path in the axial-flow pump issmaller than the conventional axial-flow pump, the axial-flow pump issuited for such an application that accepts an increase in frictionalloss.

When the tip diameter Dt is decreased while the hub diameter Dh is heldconstant (second manner), there is obtained an axial-flow pump that issmaller in size than the conventional axial-flow pump while having ahigh boss ratio and a stable pressure characteristic. In this case, thecross-sectional area of a fluid flow path in the axial-flow pump becomessmaller than in the case of the first manner.

When both of the hub diameter Dh and the tip diameter Dt are increasedin such a manner that the hub diameter Dh is increased at a higher ratethan the tip diameter Dt (third manner), there is obtained an axial-flowpump that is larger in the cross-sectional area of a flow path than theconventional axial-flow pump while having a high boss ratio and a stablepressure characteristic. In this case, since a large tip diameter Dt isselected, the size of the axial-flow pump becomes large to some extent.

When both of the hub diameter Dh and the tip diameter Dt are decreasedin such a manner that the tip diameter Dt is decreased at a higher ratethan the hub diameter Dh (fourth manner), there is obtained anaxial-flow pump that is smaller in size than the conventional axial-flowpump while having a high boss ratio and a stable pressurecharacteristic. In this case, the cross-sectional area of a fluid flowpath in the axial-flow pump becomes smaller than in the case of thethird manner.

Furthermore, by means of increasing both of the hub diameter Dh and thetip diameter Dt, a higher boss ratio can be selected while thecross-sectional area of a flow path in an axial-flow pump is heldunchanged. For example, in the case of the hub diameter Dh=7 mm and thetip diameter Dt=10 mm, the boss ratio is 0.7, and the cross-sectionalarea of the flow path is about 51π mm² (=10²π−7²π) . By contrast, in thecase of the hub diameter Dh=9.64 mm and the tip diameter Dt=12 mm, theboss ratio is 0.803, and the cross-sectional area of the flow path isabout 51π mm² (=12²π−9.64²π). In other words, the-boss ratio-can beincreased while the cross-sectional area of a fluid flow path is heldunchanged. In this case, since the larger tip diameter Dt is selected,the size of the axial-flow pump becomes larger to some extent.

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, thepresent invention may be practiced otherwise than as specificallydescribed herein.

1. An axial-flow pump for propelling fluid under pressure by means ofimpeller vanes, the axial-flow pump having such a boss ratio as tosuppress biased flow of the fluid caused by reverse flows arising in anouter circumferential portion of an inlet region of the impeller vanesand reverse flows arising in an inner circumferential portion of anoutlet region of the impeller vanes.
 2. An axial-flow pump according toclaim 1, wherein the boss ratio is 0.65 to 0.85.
 3. An axial-flow pumpcomprising a housing, an impeller provided rotatably in the housing, anda drive mechanism for rotating the impeller, and adapted to axiallypropel fluid under pressure by means of the drive mechanism rotating theimpeller, wherein the drive mechanism is fixedly attached to adownstream stationary member which, in turn, is fixedly attached to adiffuser located downstream of the impeller and projecting from an innerwall surface of the housing; the impeller comprises a rotary shaftcoupled with the drive mechanism and impeller blades projecting from anouter circumferential surface of the rotary shaft; and a boss ratio,which is the ratio between an outside diameter of the rotary shaft andan outside diameter of the impeller vanes, is 0.65 to 0.85.